The present invention relates to a ball bearing for supporting a spindle which is incorporated into a hard disk drive unit (HDD), a video tape recorder (VTR), or a digital audio tape recorder (DAT) and can be rotated at a high speed.
For example, in the case of an HDD to be incorporated into a computer, such a drive motor as shown in FIG. 1 is incorporated into the HDD and is used to drive and rotate a spindle 1 which is used to drive the HDD. Between the outer peripheral surface of the spindle 1 and the inner peripheral surface of a housing 2, there are interposed a pair of ball bearings 3, 3. Each of the bearings has such a structure as shown in detail in FIG. 2. The ball bearings 3, 3 support the spindle 1 on the inside of the housing 2 in such a manner that the spindle 1 can be rotated freely. Each of the ball bearings 3, 3 comprises a steel-made inner race 5, a steel-made outer race 7 and a plurality of steel-made balls 8, 8. The steel-made inner race 5 has an inner race raceway 4 formed on the outer peripheral surface thereof. The steel-made outer race 7 has an outer race raceway 6 formed on the inner peripheral surface thereof. The plurality of steel-made balls 8, 8 are rotatably interposed between the inner race and outer race raceways 4 and 6. A retainer 9 rotatably retains these balls 8, 8. While the balls 8, 8 are assembled to the drive motor, they are given preloads to thereby prevent them from playing when the spindle 1 is rotated.
In the case of the above-structured ball bearing 3 to be incorporated into the rotation support portion of the HDD, in view of the fact that it is used in the house interior, the ball bearing 3 is requested to be able to minimize the noises that are produced while it is in operation. In addition, at the same time, due to the enhanced density (increased storage capacity) and reduced size of the HDD in recent years, the ball bearing is requested to be able to minimize vibrations such as NRRO (Rotation asynchronous vibration). On the other hand, due to spread of OA equipment such as a notebook computer, in order to prevent trouble caused by impact loads that could be generated when the OA equipment is dropped because of wrong handling, the ball bearing 3 is further requested to be strong (impact-resistant) against the impact loads.
In order to enhance the impact resistance of the ball bearing 3 for this purpose, conventionally, there is known a technique to reduce the ratio of the radiuses of curvature of the inner race raceway 4 and outer race raceway 6 to the radiuses of curvature of the rolling surfaces of the respective balls 8, 8. (That is, there is known a technique to set the ratio of the radiuses of curvature of the inner race raceway 4 and outer race raceway 6 to the radiuses of the outside diameters of the respective balls 8, 8 slightly larger than 0.5).
Namely, the ratio of the radiuses of curvature of the inner race raceway 4 and outer race raceway 6 to the radiuses of curvature of the rolling surfaces of the respective balls 8, 8 are reduced. Contact ellipses existing on the contact portions between the raceways 4, 6 and balls 8, 8 are therefore increased in size, to thereby lower the surface pressures of these contact portions. Accordingly, even the above impact loads are not be able to produce such impressions in the raceways 4, 6 that give rise to the above-mentioned vibrations.
However, a test conducted by the inventors et al. shows that, in case where the ratio of the radiuses of curvature of the inner race raceway 4 and outer race raceway 6 to the radiuses of curvature of the rolling surfaces of the respective balls 8, 8, is reduced, the rotation torque of the ball bearing 3 is increased. At that time, as a result of this, not only the power consumption of the OA equipment incorporating such ball bearing 3 therein is increased but also the ball bearing 3 is not be able to offer sufficient impact resistance. In other words, our test shows that, in case where the above-mentioned ratio of the radiuses of curvature is reduced, the static load rating increases but the impact resistance cannot be always enhanced. For example, when the inventors et al. conducted a drop test in conformity to a case where compact OA equipment is dropped, in the case of the ball bearing with the radius of curvature ratio reduced, the acoustics of the ball bearing were degraded. It is believed that such degraded acoustics were caused by the impressions formed after the OA equipment was dropped down onto the floor.
Specifically, as to the reason why such impressions were produced, the inventors believed as follows. In the case of the ball bearing 3 with the radius of curvature ratio reduced, the rolling surfaces of the balls 8, 8 are easy to run onto the shoulders of the inner race raceway 4 and outer race raceway 6 due to the impact load applied to the ball bearing 3. Further, in case where the rolling surfaces of the balls 8, 8 run onto the raceway shoulders, impressions are produced on the rolling surfaces of the balls 8, 8 due to excessive surface pressures caused by edge loads.
In view of the above, the inventors conceived that, by increasing the above-mentioned radius of curvature ratio as well as by reducing an interior geometric clearance, there can be realized a ball bearing 3 having a following feature. That is, in the ball bearing 3, the rolling surfaces of the balls 8, 8 can be made hard to run onto the shoulders of the inner race raceway 4 and outer race raceway 6, the impact resistance thereof is enhanced.
That is, it is an object of the invention to provide such improved ball bearing.
In attaining the above object, according to the invention, there is provided a ball bearing which comprises an inner race 5, an outer race 7 and a plurality of balls 8, 8. The inner race 5 includes on the outer peripheral surface thereof an inner race raceway 4 having an arc-shaped section. The outer race 7 includes on the inner peripheral surface thereof an outer race raceway 6 having an arc-shaped section. The plurality of balls 8, 8 are rollably interposed between the inner race raceway 4 and outer race raceway 6, and a retainer 9 for holding the balls 8, 8 in such a manner that they are able to roll freely.
Especially, according to a first aspect of the invention, there is provided a ball bearing 3, wherein the ratio ri/D8 of the radius of curvature ri of the inner race raceway 4 to the outside diameter D8 of the balls 8, 8 is set in the range of 0.51xcx9c0.54 (this is, in the range from 0.51 to 0.54 with both inclusive). Also, the ratio re/D8 of the radius of curvature of the outer race raceway 6 to the outside diameter D8 of the balls 8, 8 is set in the range of 0.54xcx9c0.56 (this is, in the range from 0.54 to 0.56 with both inclusive). Further, the interior geometric clearance c is set at 13 xcexcm or less.
By the way, the interior geometric clearance c is a value that can be obtained by subtracting the diameter D4 of the inner race raceway 4 and the double of the diameter D8 of the balls 8, 8 from the diameter D6 of the outer race raceway 6 (that is, D6xe2x88x92D4xe2x88x922D8). The above-mentioned value (13 xcexcm or less) is effective especially in a miniature ball bearing in which the inside diameter R5 of the inner race 5 is 2.6 mm or less, and the diameter D8 of the balls 8, 8 is 2 mm or less.
Note that in the specification, as shown in FIG. 2, the diameter D6 of the outer race raceway 6 means the largest diameter of the outer race raceway 6 that is defined at the deepest position of the arc-shaped section thereof. Further, the diameter D4 is the smallest diameter of the inner race raceway 4 that is defined at the deepest position of the arc-shaped section thereof.
Also, according to a second aspect of the invention, there is provided a ball bearing 3, wherein the ratio ri/D8 of the radius of curvature ri of the inner race raceway 4 to the outside diameter D8 of the balls 8, 8 is set at 0.51 or more, the ratio re/D8 of the radius of curvature re of the outer race raceway 6 to the outside diameter D8 of the balls 8, 8 is set at 0.54 or more, and these ratios ri/D8 and re/D8 are respectively set in the range that can satisfy the relationship (ri/D8)=(0.647xcx9c0.662)xc3x97(re/D8)0.385. (that is, these ratios ri/D8 and re/D8 are respectively set in the range that can satisfy an equation of: 0.647xc3x97(re/D8)0.385xe2x89xa6(ri/D8)xe2x89xa60.662xc3x97(re/D8)0.385.) Further, the interior geometric clearance c is set in the range that can satisfy the relationship 0.003 less than (c/D8) less than 0.01.
According to the above-structure ball bearing 3 of the invention, the impact resistance thereof can be enhanced and thus, it is possible to realize compact OA equipment which, even in case where it is unexpectedly dropped down onto the floor of a building, is hard to be seriously damaged. By the way, in a ball bearing according to the prior art, since the outer race raceway 6 is concavely curved in the circumferential direction thereof, the contact ellipse existing in the contact portion between the outer race raceway 6 and balls 8, 8 becomes large and thus, in case where the edge of the contact ellipse and the edge of the outer race raceway 6 are near to each other, the contact portion is caused to reach the edge of the outer race raceway 6 due to the impact load applied to the ball bearing, so that the contact ellipse can be easily cut away.
On the other hand, in the case of a ball bearing 3 according to the first aspect of the invention, because the ratio re/D8 of the radius of curvature re of the outer race raceway 6 to the outside diameter D8 of the balls 8, 8 is set large in the range of 0.54xcx9c0.56 and also because the interior geometric clearance c is set small, that is, 13 xcexcm, the distance between the edge of the contact ellipse and the edge of the outer race raceway 6 is set large. Thanks to this, even in case where the impact load is applied to the ball bearing, the contact portion is made hard to reach the edge of the outer race raceway 6 and thus the contact ellipse is made hard to be cut away. On the other hand, since the inner race raceway 4 is curved projectingly with respect to the circumferential direction thereof, a contact ellipse existing in the contact portion between the inner race raceway 4 and balls 8, 8 is relatively small; but, because the ratio ri/D8 of the radius of curvature of the inner race raceway 4 to the outside diameter D8 of the balls 8, 8 is set large in the range of 0.51xcx9c0.54, there can be secured the sufficient area of the contact ellipse, thereby being able to prevent the surface pressure of the contact ellipse from increasing. In this case as well, since the distance between the edge of the contact ellipse and the edge of the inner race raceway 4 can be set large because the geometric clearance c is set small, even in case where the impact load is applied to the ball bearing 3, the contact portion is made hard to reach the edge of the inner race raceway 4 and thus the contact ellipse is made hard to be cut away. Thanks to these characteristics, as described above, the impact resistance of the ball bearing 3 can be enhanced.
Also, according to the ball bearing 3 as set forth in the second aspect of the invention, the impact resistance of the ball bearing 3 can be similarly enhanced. By the way, description will be given later in detail of the process in which the ball bearing 3 according to the second aspect of the invention is developed.
Also, in the present invention, the dimensional range where the impact resistance of the ball bearing 3 according to the first aspect of the invention can be enhanced is overlapped in part with the dimensional range where the impact resistance of the ball bearing 3 according to the second aspect of the invention can be enhanced; and, the overlapped range is a more preferable range in which the impact resistance of the ball bearing 3 can be enhanced.